Setup: (in case it matters)
Amps: 2 x GFA 555 II
Preamp: GFP 565 preamp in Bypass
Source: It depends, a computer: Gigabyte Aorus Pro AX with AMP-UP Audio, an Adcom GCD 575 or a lightly-modded Pioneer PL 512 turntable.
Speakers: Polk Audio Monitor 12 series-2 (8 Ohm speakers, still running fine)
One of the amps blew a main fuse a couple of months ago, so I just removed it and powered the setup with one of these in stereo. Sounds OK, but I'm wanting the other amp back in service. I have the time, so I pulled the cover to see if anything looked amiss (bulging caps, melting etc)....and it looks just like the other one that's running fine.
I'm looking for advice, since I'm learning as I go. I have experience in home wiring and automotive wiring. I'm generally capable with a multimeter. This is not the same, but I have time, patience and ability to learn.
I got a complete schematic and parts list,, and I'm admittedly not the best with schematics, but what I see points to output transistors and/or bridge rectifier(s). I'm prepared to pull each and test. But! Before I pull them, is there anything I should know? From my experience with old cars, I know that some things are well known in the community but not so much to "newbies". Just wanting to not make more trouble for myself.
Thanks!!
Mark
Amps: 2 x GFA 555 II
Preamp: GFP 565 preamp in Bypass
Source: It depends, a computer: Gigabyte Aorus Pro AX with AMP-UP Audio, an Adcom GCD 575 or a lightly-modded Pioneer PL 512 turntable.
Speakers: Polk Audio Monitor 12 series-2 (8 Ohm speakers, still running fine)
One of the amps blew a main fuse a couple of months ago, so I just removed it and powered the setup with one of these in stereo. Sounds OK, but I'm wanting the other amp back in service. I have the time, so I pulled the cover to see if anything looked amiss (bulging caps, melting etc)....and it looks just like the other one that's running fine.
I'm looking for advice, since I'm learning as I go. I have experience in home wiring and automotive wiring. I'm generally capable with a multimeter. This is not the same, but I have time, patience and ability to learn.
I got a complete schematic and parts list,, and I'm admittedly not the best with schematics, but what I see points to output transistors and/or bridge rectifier(s). I'm prepared to pull each and test. But! Before I pull them, is there anything I should know? From my experience with old cars, I know that some things are well known in the community but not so much to "newbies". Just wanting to not make more trouble for myself.
Thanks!!
Mark
No need to remove anything at this point.
If the mains input fuse has blown then basic checks are to read across each diode in the bridge and make sure none are dead short. Use the diode test range for these checks.
If it is a rail fuse that has blown then check across collector and emitter of each output transistor and make sure none read dead short. Also if a rail fuse has blown then make sure the reservoir caps are not charged... they hold a lot of energy.
If the mains input fuse has blown then basic checks are to read across each diode in the bridge and make sure none are dead short. Use the diode test range for these checks.
If it is a rail fuse that has blown then check across collector and emitter of each output transistor and make sure none read dead short. Also if a rail fuse has blown then make sure the reservoir caps are not charged... they hold a lot of energy.
Thanks for the input and GREAT point about the caps!
It's currently a main fuse issue, but several months before it blew the main fuse, it also blew a rail fuse. I should have checked it then, but I was driving the amp rather hard in bridged mode, so figured it was "Normal". Live and learn I suppose
I'll start with the easiest (Diode check) then move to the transistors. There's no rush, and I'm learning, so this is going to be a good exercise I imagine.
Mark
It's currently a main fuse issue, but several months before it blew the main fuse, it also blew a rail fuse. I should have checked it then, but I was driving the amp rather hard in bridged mode, so figured it was "Normal". Live and learn I suppose
I'll start with the easiest (Diode check) then move to the transistors. There's no rush, and I'm learning, so this is going to be a good exercise I imagine.
Mark
Make sure there IS something wrong before pulling anything. Put in fresh fuses and see if it comes up ok on a dim bulb. Variac if you have one. But don’t just “plug it in”. If you blew a rail fuse from just playing hard, others may have been stressed and blow early next time and you need this simple check first. (I used to DJ with Phase Linears in high school and college and the fuses would nuisance blow all the time running that hard. Replace, check on a dim bulb, then put the amp back in service.) Compare to the good amp to see how “dim” the dim bulb should be. If the amp is dead or if you’re drawing more current than the other one then look at the rectifier, caps, output transistors, and ultimately every transistor and resistor all the way back to the bias stack - depending on what you find.
So far I found a rectifier diode on the right side is bad. The last diode in the rectifier block going clockwise from node reads 0.00 and 0.00, so that's failed open. The left side reads all 457 or OL. defending on direction, so that's good.
two questions:
1. Should I replace them both diodes at the same time? 32 year old diode replacement sounds like best practice to me.
2. Before I try to power this up.....what would cause this? This is early in the current path, it seems to me that maybe running these so hard in bridged mono led to overstress/heat?
Thanks for the tips so far!!
Mark
two questions:
1. Should I replace them both diodes at the same time? 32 year old diode replacement sounds like best practice to me.
2. Before I try to power this up.....what would cause this? This is early in the current path, it seems to me that maybe running these so hard in bridged mono led to overstress/heat?
Thanks for the tips so far!!
Mark
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0.00 means dead short. On the diode range your meter is actually showing the volt drop that occurs as a small test current is passed through the device.
Remove the four 2.5A fuses that are in the power amp rails and repeat the check and see if you still read a short on the bridge.
I'm assuming the bridge is an encapsulated module and not four separate diodes. Typically something like a 35A 400 volt bridge should be fine. You need that high voltage rating or P.I.V. (peak inverse voltage) as it is called... and just looking at the parts list... they say 200 volt. I'd say that was marginal tbh given the high voltage the amp works on.
Bridge reccys are cheap, fit something high rated.
Given its a bridge failure (assuming the other checks are OK) then it is quite likely just to be that, a diode failure.
Remove the four 2.5A fuses that are in the power amp rails and repeat the check and see if you still read a short on the bridge.
I'm assuming the bridge is an encapsulated module and not four separate diodes. Typically something like a 35A 400 volt bridge should be fine. You need that high voltage rating or P.I.V. (peak inverse voltage) as it is called... and just looking at the parts list... they say 200 volt. I'd say that was marginal tbh given the high voltage the amp works on.
Bridge reccys are cheap, fit something high rated.
Given its a bridge failure (assuming the other checks are OK) then it is quite likely just to be that, a diode failure.
Yeah, they're in modules. I tested them while disconnected. Should I test them in situ? The original, installed part is BR254, and the service manual calls for KBPC2504. They are close, but certainly not exact. Which one should I go for?
https://www.digikey.com/en/products/detail/solid-state-inc/KBPC2504/14547940
OR
https://www.mouser.com/ProductDetail/Rectron/BR254?qs=lDTmbUD7thN6y9McaQ4/Yg==
Mark
https://www.digikey.com/en/products/detail/solid-state-inc/KBPC2504/14547940
OR
https://www.mouser.com/ProductDetail/Rectron/BR254?qs=lDTmbUD7thN6y9McaQ4/Yg==
Mark
If its short when disconnected, then that is conclusive it is faulty. The reason for saying to remove the rail fuses was just as a double check that there was not a short on the output of the bridge.
If you refit the rail fuses and then measure from each fuse to ground, you should not read a short.
The first one looks fine, the second actually shows as no stock and obsolete for me 🙂
All you need to go off are the specs, 400v and 25 or 35 amp is fine. There is nothing special about general purpose high current diodes and bridges in applications like this and there are literally dozens to choose from.
If you refit the rail fuses and then measure from each fuse to ground, you should not read a short.
The first one looks fine, the second actually shows as no stock and obsolete for me 🙂
All you need to go off are the specs, 400v and 25 or 35 amp is fine. There is nothing special about general purpose high current diodes and bridges in applications like this and there are literally dozens to choose from.
I would replace both at the same time and go with a larger current rating of 35A or 50A. The price difference is small.
Failure was likely due to both age and running hard.
@Mooly, thanks for the confirmation!
@techtool, that was also one of my thoughts. These amps were driven hard for years. I don't typically like air-conditioning, and in the summer, it's common to keep the house temperatures around 90 degrees f.
There's a fan cable for both of these amps, so I might just add a fan to each of them. Noctua makes silent 120mm fans, and that would be an easy modification.
Mark
@techtool, that was also one of my thoughts. These amps were driven hard for years. I don't typically like air-conditioning, and in the summer, it's common to keep the house temperatures around 90 degrees f.
There's a fan cable for both of these amps, so I might just add a fan to each of them. Noctua makes silent 120mm fans, and that would be an easy modification.
Mark
35 or 50 amp bridges can handle proportionally more non-repetitive surge current which happens at every switch-on (due to hungry capacitors). The can just “go” from this eventually. Higher voltage never hurts either. You pay for the higher current or voltage rating with a tenth or two more voltage drop (no big deal on 75 volt supplies).
Alright! I installed the two new bridge rectifiers (stock rating), and for the record, the heatsink grease was more like grout after 30 years, and wasn't easy to remove. It's now running fine. After warmup and bias check/adjustment, I ran it hard, in bridged-mono for ten or so minutes, and it held fine. I consider this a success.
Now I figure that I'll replace the bridge rectifiers on the other amp as a preventative measure. They're the same age and neither has ever been worked on (I'm the original owner). I'm also looking at preventive replacement of the small electrolytic capacitors in both amps, so I may have more questions later. In the meantime, are there any suggestions or "best practices" I should be aware of?
Mark
Now I figure that I'll replace the bridge rectifiers on the other amp as a preventative measure. They're the same age and neither has ever been worked on (I'm the original owner). I'm also looking at preventive replacement of the small electrolytic capacitors in both amps, so I may have more questions later. In the meantime, are there any suggestions or "best practices" I should be aware of?
Mark
Alright 🙂 👍 excellent news.
As to caps... we see so many tales of woe on here where recaps go wrong so just remember that just because a cap is old, it does not mean it is bad. Those near hot running parts are most likely to be the suspect ones.
If you really want to do a recap then first look at the board and make sure all are fitted in accordance with the board markings. Its not uncommon to have a board marked incorrectly and/or a circuit diagram with an error. So fit like for like and the same way as what is there now. Stick to the same values and voltage ratings (you can go higher in voltage) and make sure they are physically suitable and not too big or with the lead pitch the wrong size.
As to caps... we see so many tales of woe on here where recaps go wrong so just remember that just because a cap is old, it does not mean it is bad. Those near hot running parts are most likely to be the suspect ones.
If you really want to do a recap then first look at the board and make sure all are fitted in accordance with the board markings. Its not uncommon to have a board marked incorrectly and/or a circuit diagram with an error. So fit like for like and the same way as what is there now. Stick to the same values and voltage ratings (you can go higher in voltage) and make sure they are physically suitable and not too big or with the lead pitch the wrong size.
@Mooly, good points! I have extensive experience working on cars, where running changes and substitutions are common. I hadn't thought it would be common in the electronics world.....
My plan is to leave the 15000Uf caps and just replace the ones on the input and output boards since they get rather warm.
Lucky for me, neither amp has been altered in any way (other than the rectifiers). I figure that I'll take a photo of each part, the board layout, and refer to the service manual to verify. Lastly I'll take a few measurements and go from there. During a quick glance this morning at Mouser electronics looking at caps, I already ran into seeing the size differences. Luckily, these amps are spacious inside.
My plan is to leave the 15000Uf caps and just replace the ones on the input and output boards since they get rather warm.
Lucky for me, neither amp has been altered in any way (other than the rectifiers). I figure that I'll take a photo of each part, the board layout, and refer to the service manual to verify. Lastly I'll take a few measurements and go from there. During a quick glance this morning at Mouser electronics looking at caps, I already ran into seeing the size differences. Luckily, these amps are spacious inside.
Modern caps may actually seem a lot smaller on a like for like basis to what is fitted. Whenever I recapped something (usually an SMPS) always do it one cap at a time and make sure I know agree with the board markings before replacement. Doing it one at a time greatly reduces the risk of error rather than pulling them all out and having a lot of empty spaces to populate.